Method and system for generating CS-RZ pulses showing narrow width of bit duration

ABSTRACT

Provided are a method for processing optical signals and a transmitter for the signals adapted for an ultra-high bit rate. The method uses an optical pulse stream with a carrier suppressed return to zero (CS-RZ) format to be obtained from an optical modulator that is driven using a combination of two sinusoidal electrical voltages, one at a frequency f and the other at the third harmonic 3f of the frequency f.

TECHNICAL FIELD

The present invention relates to a method for processing optical signalsto be transmitted through an optical transmission line using an opticalpulse stream with a carrier suppressed return to zero format to beobtained from an optical modulator. Furthermore, it is related to atransmitter for optical signals to be forwarded through an opticaltransmission line, the transmitter comprising a circuit with an opticalmodulator for generating an optical pulse stream having a carriersuppressed return to zero format. The invention is based on a priorityapplication EP 04 290 658.6 which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

In order to improve the quality of Dense Wavelength Divisionmultiplexing (DWDM) transmission dedicated for long haul distances (morethan 1000 kilometers), different solutions have been proposed based onthe use of modulation format such as Non Return-to-Zero (NRZ),Return-to-Zero (RZ), Carrier-Suppressed Return-to-Zero (CS-RZ),Return-to-Zero Differential Phase Shift Keyed Signals (RZ-DPSK), PhaseShaped Binary Transmission (PSBT) etc. . . . All these techniques try toreduce the impact of propagation effect occurring usually in DWDMenvironment among other by reducing the spectral width of each channel.

In conventional manner, the power spectrum density of RZ optical signalis relatively broad because of the large number of transitions in thesignal to be transmitted. Due to the spread over a wide range offrequencies of the transmitted energy, an RZ signal is sensitive togroup velocity dispersion i.e. to chromatic dispersion, and also tofour-wave mixing (FWM) or “cross-talk” in DWDM systems. Nevertheless, RZformat presents the advantage of being little affected by self-phasemodulation (SPM) in comparison to a NRZ format. It often happens thatthe SPM induced by optical non-linearities in a line fiber gives rise tooptical signal distortion that reduces the range and the capacity ofoptical transmission systems. In addition, RZ signals are suitable forbeing regenerated by synchronous modulation.

Conversely, the power spectrum density of a NRZ optical signal isnarrower than that of a RZ signal. However, in NRZ format, both capacityand transmission range are limited by SPM. Furthermore, no optical orelectronic regenerators exist that are capable of processing suchsignals at high bit rates. Such signals are not easily integrable andintroduce losses because of the interaction between successive “0” and“1” bits, and/or distortion, so that the extinction ratio of the signalafter electrical filtering is degraded. There exist also CS-RZ opticalsignals having the property of presenting bits that are alwaysphase-shifted by 180° relative to adjacent bits. Such CS-RZ signalspossess numerous advantages over conventional signals like RZ and NRZ.More particularly, the interaction between adjacent bits are reduced dueto the different phase between neighbor bits. Therefore, the use ofCS-RZ reduces intrachannel effects, one of the main limitation foroptical transmission rates at or above 40 Gbit/s.

In an article entitled “40 Gbit/s L-band transmission experiment usingSPM-tolerant carrier-suppressed RZ format”, published in Elec. Letters,Vol. 35, No. 25, Dec. 9, 1999, p. 2213 A. Hirano et al. describe using ashifted dispersion optical fiber link in particular, a study of theoptimum dispersion stabilities between RZ, CS-RZ, and NRZ signals in thelarge (L) transmission band at frequencies in the range between 1570nanometers (nm) to 1605 nm. It appears that CS-RZ signals at 40 Gbit/spresent the most stable optimum dispersion and remain the closest to atotal dispersion in the vicinity of 0 picosecond per nanometer (ps/nm).Dispersion tolerance is explained in particular by the phase inversionbetween adjacent bits which eliminates all inter-bit interference.Furthermore, CS-RZ signals subject the sensitivity of the receiver tolittle degradation at high power. Those results also confirm that CS-RZsignals are less sensitive to SPM than are NRZ signals. In this article,the generator producing the CS-RZ optical signals at 40 Gbit/s comprisea Mach-Zehnder modulator in push-pull mode fed with a sinusoidalelectrical signal of 20 gigahertz (GHz). The CS-RZ pulse width takes 66%of the time bit (16.5 ps at 40 Gbit/s). It is usually obtained usinghalf frequency driving (typically 20 GHz) of the modulator biased at theminimum of the transfer curve.

Another type of CS-RZ clock signal generator (transmitter) is based onusing a phase modulator to change the phase of each successive bit. Dueto their limited passbands, those prior art generators or transmitter donot make it possible, at present, to produce stable CS-RZ signals at amodulation frequency exceeding 40 Gbit/s. In other words, suchgenerators or transmitters are unsuitable for producing CS-RZ signals atvery high bit rates.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a method for processing optical signals and a transmitter forthe signals adapted for an ultra-high bit rate without incurring asubstantial cost increase.

This object is achieved in accordance with the invention by applying amethod for processing optical signals to be transmitted through anoptical transmission line using optical pulse stream with a carriersuppressed return to zero (CS-RZ) format to be obtained from at least anoptical modulator. That method is characterized by driving the opticalmodulator with a combination of two sinusoidal electrical voltages, oneat some frequency f and the second one at the third harmonic 3 f of suchfrequency. It is of advantage when applying the method according to theinvention, by biasing the optical modulator at the minimum of itssinusoidal transfer function with the excitation voltage obtained fromthat combination of the two sinusoidal electrical voltages. When usingsuch a method, it may be possible to achieve a final optical pulsestream defined by CS-RZ pulses having a width of approximately 33% ofbit duration. It is possible to process such generated optical pulsestream by interleaving at least two of them differing in theirpolarisation state. Advantageously, it will then be possible to generatean optical pulse stream at frequency almost 2 f. In such a way afterapplying a bit-to-bit coding scheme to the two optical pulse streams afinal bit rate will be achieved which may exceed substantially 40Gbit/s. It is of advantage when interleaving the two optical pulsestreams to choose these two optical pulse streams with differentpolarisation state possibly perpendicular.

In an embodiment according to the invention, a transmitter for opticalsignals to be forwarded through an optical transmission line comprises acircuit with at least an optical modulator for generating such opticalpulse stream having a CS-RZ format. Such transmitter is characterized inthat the optical modulator is driven by a combination of two sinusoidalelectrical voltages, one at some frequency f and the second one at thethird harmonic 3 f of such frequency.

In an embodiment according to the invention, the transmitter comprisestwo optical paths for the generation of two optical pulse streamsdiffering in their polarisation state. Each of these optical pathscomprises an optical modulator for the insertion of data signal overthis transmitted optical light and an optical modulator according to theinvention driven by this combination of the two sinusoidal electricalvoltages. In another embodiment according to the invention, thetransmitter comprises a polarisation separating coupler placed after theoptical modulator to divide the optical pulse stream into two opticalpulse stream with different polarisation state. These two optical pulsestreams will be transmitted on different optical paths while each oftheses optical paths comprises an optical modulator for the insertion ofdata signal over the optical pulse stream. In both embodiments, the twooptical paths are converging into a coupler allowing to interleave thesetwo optical pulse streams possibly bit-to-bit.

Advantageous developments of the invention are described in thedependent claims, the following description and the drawings.

DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be explained furtherwith the reference to the attached drawings in which:

FIGS. 1 a and 1 b are diagrams of voltage excitation to be used for theoptical modulator according to the invention;

FIGS. 2 a and 2 b are diagrams of the respectively electrical andoptical output of the optical modulator according to the invention;

FIGS. 3 a and 3 b are diagrams of optical pulse streams according to theinvention;

FIG. 4 is a schematic view of a transmitter according to an embodimentof the invention;

FIG. 5 is a schematic view of another embodiment of the transmitteraccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is proposed in the present invention to apply a method for processingoptical signals using an optical pulse stream with a CS-RZ format to beobtained from an optical modulator. Such optical modulator will bedriven by a combination of two sinusoidal electrical voltages one atsome frequency f.sub.clock and the second at the third harmonic of thatfrequency 3 f.sub.clock. These two sinusoidal electrical voltages areshown on FIG. 1 a. And on FIG. 1 b is shown the electrical signal shapeobtained from that combination and which should be applied to an opticalmodulator possibly biased at the minimum of its transfer curve usuallycorresponding to its maximum extension.

As an example, in order to obtain near 16 ps pulse duration at 20 GHz,it is proposed to drive the optical modulator with such a combination ofvoltages at frequencies equal respectively to 10 GHz and 30 GHz.

On FIGS. 2 a and 2 b are shown respectively the electrical and opticaloutput of the optical modulator possibly a Mach-Zehnder. The opticaloutput is obtained after an electrical to optical conversion. Themodulator is biased at the minimum of its sinusoidal transfer functionwith the excitation voltage corresponding to FIG. 1 b.

On FIG. 3 a is shown the final pulses stream after applying PhaseMixing-Optical Time Division Multiplexing (PM-OTDM) maintainingcross-polarized bit to bit interleaving. On FIG. 3 b is shown again moreexplicitly the association according to the invention of both thebit-to-bit polarisation interleaving technique and the optical phasemodulation technique. FIG. 3 b is a schematic description of such amodulation format in the optical domain separating the two axis of thelight polarisation TE and TM and with mark “0” and “.pi.” for coding theoptical phase.

On FIGS. 4 and 5 are shown two different embodiments of a transmitteraccording to the invention. On FIG. 4 the continuous wavelengths aredivided in two parts using a polarisation separating coupler (PSC). Mod1 and Mod 2 are driven at e.g. 20 Gbit/s only with already commercialavailable drivers producing enough large peak to peak swing voltagewell-matched to actual broadband modulators for data coding in NRZformat. The multiplexing electronics is only at 20 Gbit/s which allowsimproved low cost technology with high integration efficiency andensuring better control of power consumption. The second stage of thetransmitter as shown on FIG. 4 is devoted to the format conversion. TheMod 3 and Mod 4 are driven only using narrow band amplifier at e.g. 10GHz and 30 GHz. The output amplitude should be two times the voltage ofthe used optical modulator like a LiNbO.sub.3 Mach-Zehnder modulator(typically 8V).

On FIG. 5 is shown a second embodiment of a transmitter according to theinvention. At this second scheme, the number of optical modulator isminimized. This is obtained by applying the combination of the twosinusoidal electrical voltages at frequencies f and 3 f, respectively,on a first modulator Mod 0 provided with continuous wavelengths and onlyafterwards splitting such obtained optical pulse stream using the PSC.In that case, two optical pulse streams defined by differentpolarisation states possibly orthogonal (TE, TM) will be forwardedthrough different optical paths, where each path comprises modulatorsMod 1 and Mod 2, respectively. These modulators are used to perform thebit-to-bit coding. An additional optical delay line ODL is required onone of the two optical paths to be able to interleave precisely the twoobtained data streams at e.g. 20 Gbit/s. This ODL is not required in thefirst scheme (FIG. 4) taken into account the possibility to adjust eachelectronic signal (data or clock) using an electrical delay line.

The combination of the two techniques bit-to-bit polarisationinterleaving and alternated phase modulation reduce the intra-channelimpairments. The optical spectrum of such a format (33% of final bitduration) is narrower than classical RZ modulation scheme (50% of bitduration) or even than CS-RZ format. In such a way, nonlinear effects asFWM or crosstalk will be significantly reduced which automaticallyincrease efficiency of DWDM transmission.

1. A method for processing optical signals to be transmitted through anoptical transmission line, the method comprising: inserting a datasignal over transmitted optical light using first at least one opticalmodulator in each of at least two optical paths of the opticaltransmission line; generating at least two optical pulse streamsdiffering in their polarization states and having a carrier suppressedreturn to zero format by modulating the optical light on said at leasttwo optical paths, over which the data signal is inserted, using secondat least one optical modulator which is driven using a combination oftwo sinusoidal electrical voltage signals comprising a first voltagesignal having a frequency f and a second voltage signal having afrequency 3f that is a third harmonic of the frequency f; interleavingthe at least two optical pulse streams; and generating a final opticalpulse stream from the interleaving.
 2. The method according to claim 1,further comprising biasing the second at least one optical modulator ata minimum of its sinusoidal transfer function with an excitation voltageobtained from the combination of the two sinusoidal electrical voltagesignals.
 3. The method according to claim 1, wherein the final opticalpulse stream is defined by carrier suppressed return to zero pulseshaving a width of approximately 33% of bit duration.
 4. The methodaccording to claim 1, wherein the final optical pulse stream is definedat a frequency of approximately 2f.
 5. The method according to claim 4,wherein a bit-to-bit coding is applied to the at least two optical pulsestreams to generate the final optical pulse stream.
 6. The method ofclaim 1, wherein the interleaving is performed by using an electricdelay line in one of the at least two optical paths.
 7. A transmitterfor optical signals to be forwarded through an optical transmissionline, the transmitter comprising: first at least one optical modulatorthat inserts a data signal over transmitted optical light in each of atleast two optical paths of the optical transmission line; second atleast one optical modulator that is disposed downstream of the first atleast one optical modulator and generates at least two optical pulsestreams differing in their polarization states and having a carriersuppressed return to zero format by modulating the optical light, overwhich the data signal is inserted, wherein the second at least oneoptical modulator is driven using a combination of two sinusoidalelectrical voltage signals comprising a first voltage signal having afrequency f and a second voltage signal having a frequency 3f that is athird harmonic of the frequency f, and a coupling unit that interleavesthe at least two optical pulse streams and generates a final opticalpulse stream from the interleaving.
 8. The transmitter according toclaim 7, wherein the second at least one optical modulator is biased ata minimum of its sinusoidal transfer function with an excitation voltageobtained from the combination of the two sinusoidal electrical voltagesignals.
 9. The transmitter according to claim 7, wherein the couplingunit interleaves the at least two data streams in a bit-to-bit manner.10. The transmitter of claim 7, further comprising an electric delayline in one of the at least two optical paths for the interleaving. 11.A transmitter for optical signals to be forwarded through an opticaltransmission line, the transmitter comprising: first at least oneoptical modulator that generates a first optical pulse stream with acarrier suppressed return to zero format by applying to transmittedoptical light a combination of the two sinusoidal electrical voltagesignals comprising a first voltage signal having a frequency f and asecond voltage signal having a frequency 3f that is a third harmonic ofthe frequency f; a polarisation separating coupler (PSC) placeddownstream of the first at least one optical modulator to divide thefirst optical pulse stream into at least two optical signals withdifferent polarisation states to be transmitted on different opticalpath; second at least one optical modulator that is disposed downstreamof the PSC, and generates at least two optical pulse streams with thecarrier suppressed return to zero format and the two differentpolarisation states by inserting a data signal into a respective opticalsignal of the at least two optical signals divided by the PSC; and acoupling unit that interleaves the at least two optical pulse streamsand generates a final optical pulse stream from the interleaving. 12.The transmitter of claim 11, further comprising an optical delay line inone of the different optical paths for the interleaving.
 13. A methodfor processing optical signals to be transmitted through an opticaltransmission line, the method comprising: generating a first opticalpulse stream with a carrier suppressed return to zero format using firstat least one optical modulator that applies to transmitted optical lighta combination of the two sinusoidal electrical voltage signalscomprising a first voltage signal having a frequency f and a secondvoltage signal having a frequency 3f that is a third harmonic of thefrequency f; dividing the first optical pulse stream into at least twooptical signals; generating at least two optical pulse streams havingthe carrier suppressed return to zero format and two differentpolarisation states using second at least one optical modulator byinserting a data signal into a respective optical signal of the at leasttwo optical signals; interleaving the at least two optical pulsestreams; and generating a final optical pulse stream from theinterleaving.
 14. The method of claim 13, wherein the interleaving isperformed by using an optical delay line with respect to one of the atleast two optical pulse streams.